This invention relates to a power amplifier, and more particularly, but not solely to an audio power amplifier for driving a loudspeaker.
Audio power amplifiers are arranged to amplify a low level audio signal, say the output from a compact disc player, and to amplify it sufficiently to drive a loudspeaker. Known audio power amplifiers comprise a first, or so-called voltage gain, stage which amplifies the voltage and a second, or so-called, output stage, which amplifies the current sufficiently to drive a loudspeaker. The output power delivered to the loudspeaker is the product of the output voltage and output current.
In low power applications up to a few watts, the complete power amplifier is available as an inexpensive integrated circuit. However, for high power applications discrete components have to be used.
Loudspeakers are driven with an alternating current which varies between positive and negative values. Typically, the output stage of known power amplifiers comprises a pair of transistors which are arranged to conduct on respective half cycles of the input. A disadvantage of these so-called class B amplifiers is that cross-over distortion can occur as one transistor starts conducting and the other transistor stops conducting.
Single transistor output stages are known, however, these typically comprise an emitter follower and a constant current sink. These so-called class A amplifiers have the disadvantage that power is consumed even when no signal is applied to the input, although they do not suffer from cross-over distortion.
I have now devised an output stage for a power amplifier which alleviates the above-mentioned problems.
In accordance with this invention, there is provided an output stage for a power amplifier, the output stage comprising an amplifying device connected in series with a variable current sink between a pair of supply rails to which first and second voltage potentials are respectively applied, a load connected between a point intermediate the amplifying device and the variable current sink and a point to which a third potential is applied, said third potential lying between said first and second potentials, the circuit further comprising control means for increasing the amount of current required by the variable current sink as the amount of current delivered to the load by the amplifying device falls.
In one half cycle the load is provided with current through the amplifying device. In the other half cycle the variable current sink draws current through the load.
When no input is applied, the variable current sink draws a low current and thus the amplifier has an improved efficiency compared with conventional class A amplifiers. The voltage across the load is always controlled by the amplifying device and thus cross over distortion will not occur.
Preferably the variable current sink is controlled by a negative feedback loop which includes said current sink and said amplifying device
Preferably the amplifying device comprises a single transistor.
Preferably the transistor comprises first, second and third terminals, said third terminal being arranged to control the amount of current flowing between said first and second terminals, the first of said supply rails being connected to said first transistor terminal via resistance means, said second transistor terminal being connected to the second of said supply rails by said current sink, the load being connected to said second transistor terminal. The feedback loop comprises an input connected from a point between said resistance means and said first transistor terminal to an amplifier whose output is connected to a control terminal of the current sink.
Preferably the current sink comprises a transistor having the first connected to the amplifying device, the second terminal connected via resistance means to the second supply rail, and the third terminal being arranged to control the amount of current flowing between the first and second terminals of the transistor of the current sink.
These and other objects, features and advantages of the present invention will be clearly understood through consideration of the following detailed description.